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Date: Thu, 28 Nov 2013 12:16:59 +0200
From: Avi Kivity <avi@cloudius-systems.com>
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Subject: Re: [Qemu-devel] [RFC] create a single workqueue for each vm to
 update vm irq routing table
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Huangweidong (C)" <weidong.huang@huawei.com>, Gleb Natapov <gleb@redhat.com>, KVM <kvm@vger.kernel.org>, "Michael S. Tsirkin" <mst@redhat.com>, "Zhanghaoyu (A)" <haoyu.zhang@huawei.com>, Luonengjun <luonengjun@huawei.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Zanghongyong <zanghongyong@huawei.com>, Avi Kivity <avi.kivity@gmail.com>, Gleb Natapov <gleb@minantech.com>, "Jinxin (F)" <jinxin712@huawei.com>

On 11/28/2013 11:53 AM, Paolo Bonzini wrote:
> Il 28/11/2013 10:49, Avi Kivity ha scritto:
>> Linux is safe, it does interrupt migration from within the interrupt
>> handler.  If you do that before the device-specific EOI, you won't get
>> another interrupt until programming the MSI is complete.
>>
>> Is virtio safe? IIRC it can post multiple interrupts without guest acks.
>>
>> Using call_rcu() is a better solution than srcu IMO.  Less code changes,
>> consistently faster.
> call_rcu() has the problem of rate limiting, too.  It wasn't such a
> great idea, I think.
>
> The QRCU I linked would work great latency-wise (it has roughly the same
> latency of an rwsem but readers are lock-free).  However, the locked
> operations in the read path would hurt because of cache misses, so it's
> not good either.
>

I guess srcu would work.  Do you know what's the typical write-side 
latency there? (in terms of what it waits for, not nanoseconds).